After a long absence from Perl (alas, Java pays the bills), I was looking at getting back to competency with perl 6 and looking at a "try writing an idiomatic blackjack program." The 1 or 11 nature of the ace seems like a perfect use for a Junction. To make sure I knew how to use a Junction and sum with it, I started with a list of numbers and worked with the "most people write it this way" of https://docs.perl6.org/type/Array#(List)_routine_reduce : 
my @list = (1, 2, 3);
my $sum = [+] @list;
say $sum;

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And that worked. Adding a Junction to this: 
my @list = (1, 2|3, 4);
my $sum = [+] @list;
say $sum;

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And that spat out an error: "Type check failed in assignment to $sum; expected Numeric but got Junction (any(3, 4))"

Switching this code to another form of list summation 

my @list = (1, 2|3, 4);
my $sum = @list.reduce(*+*);
say $sum;

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That works fine.

This mismatch of expectations and errors also occurs with $sum = @list.sum; when there is a Junction in the list. So, what I'm wondering is "what is the difference between these invocations?" I suspect its something with the type system that I haven't wrapped my head around yet and that it is working as expected.

It's also quite possible that I'm introducing an XY problem in to this and that I shouldn't be doing it this way in the first place - and if that's the case, a pointer off to the more idiomatic way of dealing with what looks to be a good fit for the ace = 1|11 state would be appreciated.

In reply to [Perl6] [+] on a list... with a Junction by m_turner

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